Time of year, length of cruise, and itinerary all will have an effect. Cheap 3 and 4 day cruises will always have quite a few children. Even seven day cruises when they have great 3&4th passenger rates. I travel during non-peak times and it amazes me how many people will take their children out of school. Of course one doesn't know how many may be home schooled either.
The cruise lines provide plenty for the children to do. If children are being a nuisance it most likely is the parents fault, not the cruise lines. If things get too much of a nuisance it should be reported to the proper authority. Not the bar tenders or cabin steward, but the proper authority; security, pursers desk, or higher if need be. It serves no purpose to remain complacent and come home and complain about how unruly the kids were on a ship. But also keep in mind everyone's definition of unruly children and tolerance of children is different.